Wavelength tunable lasers are widely used as light sources in optical communication systems using wavelength division multiplexing. In a wavelength tunable laser, a wavelength locker is used to control the lasing wavelength precisely. In a typical wavelength locker, light split from the output light of a wavelength tunable laser is allowed to pass through a wavelength filter that has periodic transmission peaks, such as an etalon filter, and the intensity of the transmitted light is monitored. The wavelength is controlled via feedback so that the monitored value coincides with the intensity of the transmitted light of the etalon at a target wavelength.
In a wavelength locker, when the temperature of a wavelength filter (etalon in the above example) that has a periodic transmission characteristic changes, the transmission characteristic of the filter shifts with respect to the wavelength without changing the shape of the transmission characteristic. Thus, if the wavelength is controlled so that the monitored value of the intensity of the transmitted light coincides with a desired value, the wavelength of the light source shifts from the target wavelength in accordance with an amount of shift of the transmission characteristics of the filter caused by the temperature change, and this shift is a problem.
A wavelength monitor that is used in an optical transmission module and configured to include a first wavelength measuring unit and a second wavelength measuring unit has been proposed (for example, refer to Japanese Laid-open Patent Publication No. 2005-32968). In this wavelength monitor, the ratio of an amount of change in the transmittance with respect to a wavelength change to an amount of change in the transmittance with respect to a change in the external environment temperature differs between the first wavelength measuring unit and the second wavelength measuring unit.
In this method of the related art, the lasing wavelength of a laser is stabilized by controlling the optical transmission module so that the ratio of ΔIpd1 to ΔIpd2 coincides at all times with the ratio of a temperature variation coefficient of the etalon filter in the first wavelength measuring unit to a temperature variation coefficient of the etalon filter in the second wavelength measuring unit, where ΔIpd1 and ΔIpd2 are amounts of monitor current shift from a target value at a reference temperature, which are measured by the first wavelength measuring unit and by the second wavelength measuring unit, respectively. This leads to complex control processing. An unexpected temperature difference between the two etalon filters having different temperature characteristics may be caused, and thus it is difficult to extract only an effect of the difference between the temperature characteristics of the two periodic filters. In view of the foregoing discussion, a wavelength filter to control a shift in the wavelength of a light source preferably detects the wavelength of the light source irrespective of the ambient temperature change.